Electrochemical reduction of CO2 to CH4 and C2 H4

ABSTRACT

A process for electrochemical reduction of CO 2  to CH 4  and C 2  H 4  providing both high current densities and high Faradaic efficiencies. The process is carried out in an electrochemical cell wherein copper is electrodeposited in situ on the cathode surface making freshly deposited copper available for the electrochemical reduction. Faradaic efficiencies of about 75 to about 98 percent for production of CH 4  and C 2  H 4  are obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for electrochemical reduction of CO₂to CH₄ and C₂ H₄ providing both high current densities and high Faradaicefficiencies. The process is carried out in an electrochemical cellwherein copper is electrodeposited in situ on the cathode surface duringat least initial cell operation. Faradaic efficiencies of 73 percent forCH₄ and 25 percent for C₂ H₄ at a current density of 8.3 mA/cm² havebeen obtained.

2. Description of the Prior Art

Considerable effort has been expended towards promoting theelectrochemical reduction of CO₂ to useful hydrocarbons at both highFaradaic efficiencies and high current densities. While a number ofchemical catalysts have been identified for CO₂ reduction to methane andhigher hydrocarbons in the gas phase, relatively few catalysts have beenidentified for electrochemical reduction of CO₂ to hydrocarbons in anaqueous electrolyte.

Indirect reduction of CO₂ on a mercury electrode in an aqueouselectrolyte, pH 7, containing TiCl₃, Na₂ MoO₄ and pyrocatechol where thetotal Faradaic efficiency for cathodic hydrocarbon generation was about0.2 percent at 7 mA/cm², with methane being the major hydrocarboncomponent, is taught by Petrova, G. N. and O. N. Efimova,Elektrokhimiya, 19(7), 978 (1983). CO₂ has been shown to be reducible toCH₄, CO, and methanol at ruthenium cathodes in CO₂ saturated aqueous Na₂SO₄ electrolyte with Faradaic efficiencies for CH₄ production up to 42percent at current densities up 0.11 mA/cm² by Frese, Jr., K. W. and S.Leach, J. Electrochem. Soc., 132, 259 (1985).

Copper, 99.99 percent pure, was used as a cathode with 0.5M KHCO₃electrolyte for the electrochemical reduction of CO₂ at ambienttemperature and current density of 5.0 mA/cm² for 30 to 60 minutes withFaradaic efficiencies for CH₄ of 37 to 40 percent, Hori, Y, K. Kikuchiand S. Suzuki, Chem. Lett., 1695 (1985). In later work, high puritycopper cathodes, 99.999 percent, were used for the electrochemicalreduction of CO₂ in 0.5M KHCO₃ electrolyte in a cell operated at acurrent of 5 mA/cm² for 30 minutes at temperatures of 0° C. and 40° C.,shows Faradaic efficiency for production of CH₄ drops from 60 percent at0° to 5 percent at 4020 ; C₂ H₄ increases from 3 percent at 0° to 18percent at 40°; while hydrogen production increases from 20 percent at0° to 45 percent at 40°. It is stated that 99.99 percent pure copper cutthe Faradaic efficiencies to about one-third of those obtained with99.999 percent pure copper. Hori, Y, K. Kikuchi, A. Murata and S.Suzuki, Chem. Lett., 897 (1986). Later work of electrochemical reductionof CO₂ at a 99.999 percent pure copper cathode in aqueous electrolytesof KCl, KClO₄, and K₂ SO₄ at 19° C. and current density of 5 mA/cm⁻²showed Faradaic yields of C₂ H₄ of as high as in the order of 48percent, CH₄ 12 percent and EtOH 21 percent. Hori, Y, A. Murata,Takahashi and S. Suzuki, J. Chem. Soc., Chem. Commun, 17, 1988.

Electroreduction of CO at a 99.999 percent pure copper cathode in anaqueous catholyte of KHCO₃ at ambient temperature for 30 minutes showedhydrogen to be the predominant product, and at 1.0 mA/cm², C₂ H₄Faradaic production was 22 percent, CH₄ 1 percent; 2.5 mA/cm² C₂ H₄Faradaic production was 21 percent, CH₄ 16 percent and at 5.0 mA/cm² C₂H₄ Faradaic production was 16 percent, CH₄ 6 percent. Hori, Y, A.Murata, R. Takahashi and S. Suzuki, J. Am. Chem. Soc., 109, 5022 (1987).Similar work by the same authors showed electroreduction of CO at a99.999 percent pure copper cathode in an aqueous 0.1M KHCO₃ pH 9.6catholyte at 19° C. at 2.5 mA/cm² resulted in Faradaic production C₂ H₄of 21.2 percent; CH₄ of 16.3 percent; EtOH of 10.9 percent; and 45.5percent H₂. Hori, Y, A. Murata, R. Takahashi and S. Suzuki, Chem. Lett.,1665 (1987).

In the reduction of CO₂ to CH₄ using 99.9 percent pure cold rolled B 370copper cathodes with a CO₂ saturated 0.5M KHCO₃ electrolyte, Faradaicefficiencies of 33 percent were achieved for CH₄ at current densities upto 38 mA/cm² with no C₂ H₄ being detected. Cook, R. L., R. C. McDuff andA. F. Sammells, J. Electrochem. Soc., 134, 1873 (1987).

Electrochemical reduction of CO₂ to CH₄ and C₂ H₄ was shown to occur atcopper/Nafion electrodes (solid polymer electrolyte structures) atFaradaic efficiencies of about 9 percent for each CH₄ and C₂ H₄ atE=-200V vs. SCE using 2 mM H₂ SO₄ counter solution at a temperature of22° C. Dewulf, D. W., A. J. Bard, Cat. Lett. 1, 73, (1988).

The CO₂ reduction has previously been indicated to be highly dependentupon platinum electrode surface morphology in the production of HCOOH.Czerwinski, A., J. Sobkowski and R. Marassi, Anal. Lett., 18, 1717(1985). Simultaneous in situ deposition of nickel as an electrocatalystin the electrochemical hydrogenation of organic molecules has proveneffective for obtaining high activity catalytic sites. Lain, M. J. andD. Pletcher, Electrochim. Acta., 32, 99 (1987).

SUMMARY OF THE INVENTION

The process of this invention provides electrochemical reduction of CO₂to CH₄ and C₂ H₄ at both high current densities and high Faradaicefficiencies. Faradaic yields of hydrocarbons by the electrochemicalreduction of CO₂ according to this invention can be in order of 98percent at 8.3 mA/cm² and about 79 percent at an increased currentdensity of 25 mA/cm². We have found that to obtain such high Faradaicyields at high current densities by the electrochemical reduction ofCO₂, it is important to provide a cathode surface of in situ depositeduniformly granular copper over the entire cathode substrate. Suitable insitu copper deposition may be achieved in any suitable electrolytic cellwherein the cathode substrate is a suitable electrically conductingmetal substance upon which copper can be deposited immersed in anaqueous inorganic salt electrolyte in which CO₂ is soluble andcomprising a copper cation supply material which will form coppercations under electrolytic cell operating conditions without interferingwith the anodic reaction. In preferred embodiments, glassy carbon is asuitable cathode substrate material, KHCO₃ is a suitable aqueouselectrolyte, and CuSO₄ is a suitable copper cation supply material.Cathode surface copper can be continuously or intermittently depositedduring the CO₂ reduction process or the cathode copper surface can beperiodically regenerated by anodic polarization followed by copperredeposition.

DESCRIPTION OF PREFERRED EMBODIMENTS

The process for electrochemical reduction of CO₂ to CH₄ and C₂ H₄ athigh current densities and high Faradaic efficiencies may be conductedin any suitable electrochemical cell configuration wherein the cellcomprises an anode and a cathode in contact with an electrolyte andmeans for passing a current between the anode and cathode. The anode maybe any suitable electrically conducting metal substance suitable foreffective electrolytic cell operation, such as platinum, nickel, lead,glassy carbon, Ebony and titanium, preferably nickel, glassy carbon andlead. Suitable cathodes include any electrically conducting metalsubstrate upon which copper may be deposited. Suitable cathodesubstrates include glassy carbon, copper and metals of the 3d, 4d and 5dtransition series, preferably glassy carbon and copper. To obtain evenand complete electrode position of copper granules on the surface of thecathode, it is preferred that the cathode surface be highly polished byany suitable means known to the art, such as by very fine, 0.05 micron,alumina paste.

Any aqueous inorganic salt solution in which CO₂ is soluble and whichdoes not provide interfering ions may be used as an electrolyte, such asaqueous solutions of KHCO₃, NaHCO₃, KCl, KClO₄, KOH, KBF₄, K₂ CO₃, K₂SO₄, KHSO₄, KH₂ PO₄, K₂ HPO₄, preferably KHCO₃ or NaHCO₃ inconcentrations of about 0.3 to 0.8 Molar, preferably about 0.4 to 0.6Molar at pH preferably of about 4 to about 9. However, ammoniacontaining compounds and tetraalkyl cations must be avoided. Theelectrolyte also comprises a suitable copper ion supply material, whichis any inorganic copper salt which will form copper cations underelectrolytic cell operating conditions without interfering with theanodic reaction, such as CuSO₄, Cu(NO₃)₂, Cu(BrO₃)₂ and Cu(BO₂)₂. Theelectrolyte has a high content of dissolved CO₂ and is preferablysaturated with CO₂, at least in the region of the cathode.

While the process of this invention may be conducted in a singleelectrolyte cell, it is preferred that the process be conducted in aseparated cell wherein the separator is any suitable hydrogen ionpassing membrane, such as Nafion 417, Nafion 117, fiber cloth comprisingfibers of glass or polypropylene or PVC or Teflon or Nylon, porousplastics of polyethylene or PVC or Teflon. When a separatedelectrochemical cell is used, the above electrolytes are suitable foruse as catholytes, and the copper supply material is within thecatholyte which provides better control of the operating cell fordesired intimate contact of the CO₂ with the freshly in situ depositedcopper cathode surface. When the separated electrochemical cell is used,the anolyte may be different from the catholyte, but preferably, theanolyte is the same as the catholyte but does not contain the coppersupply material nor the CO₂. Flowing catholytes or electrolytes may beused to more readily provide extra cellular chemical treatment orcontrol of the catholyte or electrolyte.

We have found in the electrochemical reduction of CO₂ to CH₄ and C₂ H₄according to the present invention that Faradaic yields for CH₄ and C₂H₄ are little affected over an electrolyte pH range of about 9 to 6.5.Conduct of the electrochemical reduction at current densities of 10 to55 mA/cm² shows a peak of about 7 times the Faradaic efficiency for CH₄as for C₂ H₄ at 0°, while reversing itself to a peak of C₂ H₄ productionabout 2 times that of CH₄ production at 27° C. At both 0° C. and 27° C.,the dependency of Faradaic efficiency on current density went through acommon maximum for both CH₄ and C₂ H₄ with the peak at 0° being at acurrent density of about 25 mA/cm² and about 30 mA/cm² at 27° C.

While we do not wish to be bound by any mechanism for the process ofthis invention, our work indicates the reduction of CO₂ to CH₄ and C₂ H₄may follow the reaction path:

1. Electron transfer step to CO₂ adsorbed on cathode:

    CO.sub.2 ads +e.sup.- →CO.sub.2 ads

2. Electron transfer forming adsorbed formic acid: ##STR1## 3. Reductionof adsorbed formic acid:

    HCOOH.sub.ads +e.sup.- →HCOOH.sub.ads

followed by

    HCOOH →.CHO+OH.sup.- or

    HCOOH →HCOO.sup.- +H.sup..

and an additional side reaction

    H.sup.. +HCO.sub.2.sup.- →H.sub.2 +CO.sub.2

4. Formation of product precursors:

    HCOH.sub.ads +e.sup.- →CH.sub.ads +OH.sup.-

    CO→C.sub.ads +O.sub.ads

    C.sub.ads +H.sub.ads →CH.sub.ads

    HCOH⃡HCHO (isomerization)

5. CH₄ and C₂ H₄ formation:

    CH.sub.ads +H.sub.ads →CH.sub.2 ads

    CH.sub.2 ads +H.sub.ads →CH.sub.3 ads

    CH.sub.3 ads +H.sub.ads →CH.sub.4

    2CH.sub.2 →C.sub.2 H.sub.4

Further reaction pathways may lead to chain growth and formation ofethanol and propanol in addition to CH₄ and C₂ H₄ according to thisinvention.

The process for electrochemical reduction of CO₂ to CH₄ and C₂ H₄ atboth high current densities and high Faradaic efficiencies is achievedby provision of a suitable electrochemical cell having a cathode of anyelectrically conducting substrate upon which copper can be deposited anda surrounding aqueous inorganic salt electrolyte in which CO₂ is solubleand comprising a copper supply material which will form copper cationsunder electrolytic cell operating conditions without interfering withthe anodic reaction. A current providing a current density of about 5 toabout 50 mA/cm² is passed between the anode and cathode electrodepositing copper ions from the electrolyte forming granular copper on thecathode substrate surface in situ. The cathode substrate surface iscovered with finely divided copper particles following about 5 to 15minutes cell operation. Cathode surface copper electrode position maytake place continuously or intermittently during the CO₂ reductionprocess or the entire cathode copper surface may be periodicallyregenerated by anodic polarization followed by copper redeposition toassure fresh in situ deposited copper surface of the cathode substrate.The CO₂ gas may then be passed through the electrolyte in the proximityof the in situ electrodeposited copper cathode surface wherein at leasta portion of the CO₂ is reduced to CH₄ and C₂ H₄ at the in situdeposited copper cathode surface, as more fully described above. Gaseousproducts comprising CH₄ and C₂ H₄ are removed from the electrolyte inthe region of the cathode and may be separated or further treated asdesired in any extra cellular process.

The following examples set forth specific materials and processconditions in detail and are only intended to exemplify the inventionand not to limit it in any way.

EXAMPLE I

A glassy carbon (Electrosynthesis Co.) cathode used as a substrate forcopper deposition was initially polished with respectively 1, 0.3 and0.05 micron alumina paste (Alpha Micropolish II). Current collection tothe cathode was via a copper wire attached using silver epoxy (Epotec).The electrode assembly was appropriately insulated (Chem Grip epoxy) sothat only the front face (0.2 to 0.3 cm²) was exposed to theelectrolyte. All electrolyses were performed in a glass H-cell withseparation between anolyte and catholyte compartments being achieved bya Nafion 417 membrane (0.017 in, H⁺ Form, equivalent weight 1100). Theanode was platinum and 0.5M KHCO₃ was used for both anolyte andcatholyte with 5×10⁻⁴ M CuSO₄ being initially present in the catholyte.The catholyte was sprayed with CO₂ through a glass frit (25-50μ poresize) for 30 minutes prior to initiating electrolysis. The CO₂ used wasinitially passed through a hydrocarbon trap (Chemical Research Supplies,Inc.) for removal of any CH₄ traces initially present in the gas stream,and then subsequently passed through an oxygen trap (Oxy-Trap, AlltechAssociates) prior to being introduced into the electrolysis cell. Allelectrolyses were performed during continuous CO₂ catholyte sparge withthe glass frit placed in close proximity to the working electrode.Analysis of the CO₂ gas stream before and after passage through thiselectrolysis cell was performed using a GOW-MAC Model 69-750 FID gaschromatograph using a 6'×1/8" stainless steel column packed with 80/100mesh Carbosphere (Alltech Associates). No CH₄ could be detected in theCO₂ gas stream after the hydrocarbon trap and prior to the electrolysiscell. Constant-voltage and constant-current electrolyses were controlledvia a Stonehart BC 1200 potentiostat/galvanostat.

Constant current electrolyses were performed at 0° C. in the abovedescribed cell at different specified current densities set forth inTable 1. Each current density was maintained for 15 minutes prior to GCanalysis of the exit gas stream. It is probable that the majority ofcopper deposition onto the glassy carbon working electrode occurredduring the first ten minutes of electrolysis, followed by the latercontinuous deposition of remaining trace copper from the catholyte.During cell operation in situ deposited copper at the working electrodein this oxygen free electrolyte was probably cathodically protectedagainst oxide formation.

Under conditions where a uniform finely granular in situ Cu deposit wasproduced covering all of the electrode surface, the correspondingFaradaic yields for CH₄ and C₂ H₄ are summarized in Table 1 as afunction of current density.

                  TABLE 1                                                         ______________________________________                                        Current Density  Faradaic Efficiency                                          mA/cm.sup.2      CH.sub.4 (%)                                                                           C.sub.2 H.sub.4 (%)                                 ______________________________________                                         8.3             73       25                                                  16.7             70       15                                                  25.0             68       11                                                  ______________________________________                                    

As can be seen at 8.3 mA/cm² the CO₂ reduction reaction was almostFaradaic. Even at 25 mA/cm² the overall Faradaic efficiency for CO₂reduction products was 79 percent. These are by far the highest Faradaicefficiencies yet reported for this CO₂ reduction reaction. Thesignificant preliminary observation here was the importance of freshlyin situ deposited copper on the glassy carbon substrate, together withthe possibility of copper being continuously deposited as the CO₂reduction reaction proceeds, in order to achieve these high Faradaicefficiencies.

EXAMPLE II

An electrolysis cell as described in Example I except that the granularcopper deposit covered only about one-third of the surface of the glassycarbon electrode substate was used for constant current electrolysisunder the same conditions resulting in Faradaic efficiencies for CO₂reduction to CH₄ and C₂ H₄ as summarized in Table 2:

                  TABLE 2                                                         ______________________________________                                        Current Density  Faradaic Efficiency                                          mA/cm.sup.2      CH.sub.4 (%)                                                                           C.sub.2 H.sub.4 (%)                                 ______________________________________                                        10               50       11                                                  10               21       11                                                  15               15       12                                                  20               18       14                                                  25               12       14                                                  30               12       12                                                  ______________________________________                                    

The importance of a uniform granular copper deposit over the entireglassy carbon substrate can be seen by reference to results summarizedin Table 2, where a non-uniform copper deposit was present. Even underthe conditions of this example, Faradaic efficiencies for both CH₄ andC₂ H₄ were 61 percent at 10 mA/cm² but deteriorated to 24 percent at 30mA/cm². In several instances ethane was also detected at concentrations<0.1 percent.

EXAMPLE III

To ascertain possible contribution of glassy carbon from a cathode as acarbon source for methane formation, a glassy carbon cathode was used inan electrochemical cell having 0.5M KHCO₃ electrolyte saturated with CO₂in the cathode compartment. Electrolysis was conducted at 0° C. for 60minutes at a current density of 20 mA/cm². Less than 0.1 percentFaradaic yield of methane and no ethylene was detected.

Electrolysis was conducted in a similar cell with a catholyte of 0.25MK₂ SO₄ containing 5×10⁻⁴ M CuSO₄ under continuous N₂ purge at currentdensities of 10 to 50 mA/cm². No hydrocarbon products were observed bygas chromotographic analysis of the exiting gas stream. Another similarelectrolysis was conducted using 0.25M Na₂ SO₄ with the product gasshowing no hydrocarbons. These electrolyses show that deposited copperon the glassy carbon substrate does not catalyze substrate reduction toresult in gaseous hydrocarbon products.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for purpose of illustration, it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein can bevaried considerably without departing from the basic principles of theinvention.

I claim:
 1. A process for electrochemical reduction of CO₂ to CH₄ and C₂H₄ at both high current densities and high Faradaic efficiencies in anelectrochemical cell comprising an anode an a cathode in contact with anelectrolyte, said process comprising: passing a current between saidanode and said cathode; electrodepositing Cu ions form an electrolytecomprising an aqueous inorganic salt solution in which CO₂ is solubleand Cu ions forming deposited uniformly granular Cu on a highly polishedcathode surface in situ; passing CO₂ through said electrolyte andcontacting said cathode surface; reducing at least a portion of said CO₂to CH₄ and C₂ H₄ at said in situ deposited Cu cathode surface; removinggaseous products comprising CH₄ and C₂ H₄ form said electrolyte.
 2. Aprocess according to claim 1 wherein said inorganic salt is in aconcentration of about 0.3 to about 0.8 Molar and said electrolyte is ata pH of about 4 to
 9. 3. A process according to claim 1 wherein saidelectrolyte inorganic salt is selected from the group consisting ofKHCO₃, NaHCO₃, KCl, KClO₄, KOH, KBF₄, K₂ CO₃, K₂ SO₄, KHSO₄ KH₂ PO₄ andK₂ HPO₄.
 4. A process according to claim 3 wherein said Cu ions aresupplied by a copper compound selected from the group consisting ofCuSO₄, Cu(NO₃)₂, Cu(BrO₃)₂ and Cu(BO₂)₂.
 5. A process according to claim1 wherein said cathode comprises a metal substrate selected from thegroup consisting of glassy carbon, copper, and metals of the 3d, 4d and5d transition series.
 6. A process according to claim 1 wherein saidcathode comprises a metal substrate selected from the group consistingof glassy carbon and copper.
 7. A process according to claim 1 whereinsaid electrolyte is separated by an H³⁰ ion passing separator into ananolyte and a catholyte, said electrodepositing Cu ions forming granularCu on said cathode surface in situ and said passing CO₂ and contactingsaid cathode surface taking place in said catholyte.
 8. A processaccording to claim 1 wherein said current is in an amount to result incurrent densities on said cathode of about 5 to about 50 mA/cm².
 9. Aprocess according to claim 1 wherein said current is in an amount toresult in current densities on said cathode of about 20 to about 30mA/cm².
 10. A process according to claim 1 wherein said electrolyte ismaintained at a temperature about 0° to about 30° C.
 11. A processaccording to claim 1 wherein said electrolyte is maintained at atemperature about 0° to about 10° C. for preferential CH₄ production.12. A process according to claim 1 wherein said electrolyte ismaintained at a temperature about 20° to about 30° C. for preferentialC₂ H₄ production.
 13. A process according to claim 1 wherein saidgranular Cu is continuously formed on said cathode surface to providefresh in situ deposited Cu.
 14. A process according to claim 1 whereinsaid granular Cu is intermittently formed on said cathode surface toprovide fresh in situ deposited Cu.
 15. A process according to claim 1wherein said granular Cu cathode surface is periodically regenerated byanodic polarization followed by said electrodepositing Cu ions forminggranular Cu on said cathode surface in situ to provide fresh in situdeposited Cu.
 16. In a process for electrochemical reduction of CO₂ toCH₄ and C₂ H₄ at both high current densities and high Faradaicefficiencies in an electrochemical cell comprising an anode and acathode in contact with an electrolyte, wherein the improvement in thecathode half cell comprises: electrodepositing Cu ions form anelectrolyte comprising an aqueous inorganic salt solution in which CO₂is soluble and Cu ions forming in situ deposited uniformly granular Cuon a highly polished cathode surface; passing CO₂ through saidelctrolyte and contacting said in situ deposited Cu cathode surface;reducing at least a portion of said CO₂ to CH₄ and C₂ H₄ at said in situdeposited Cu cathode surface.
 17. In a process according to claim 16wherein said electrolyte is an aqueous inorganic salt solution in whichCO₂ is soluble and said inorganic salt is a concentration of about 0.3to about 0.8 Molar and said electrolyte is at a pH of about 4 to about9.
 18. In a process according to claim 17 wherein said electrolyteinorganic salt is selected from the group consisting of KHCO₃, NaHCO₃,KCl, KClO₄, KOH, KBF₄, K₂ CO₃, K₂ SO₄, KHSO₄, KH₂ PO₄ and K₂ HPO₄. 19.In a process according to claim 17 wherein said Cu ions are supplied bya copper compound selected from the group consisting of CuSO₄, Cu(NO₃)₂,Cu(BrO₃)₂ and Cu(BO₂)₂.
 20. In a process according to claim 17 whereinsaid cathode comprises a metal selected from the group consisting ofglassy carbon, copper, and metal of the 3d, 4d and 5d transition series.21. In a process according to claim 17 wherein said current is in anamount to result in current densities on said cathode of about 5 toabout 50 mA/cm².
 22. In a process according to claim 17 wherein saidcurrent is in an amount to result in current densities on said cathodeof about 20 to about 30 mA/cm².
 23. In a process according to claim 17wherein said electrolyte is maintained at a temperature about 0° toabout 30° C.
 24. In a process according to claim 17 wherein saidgranular Cu is continuously formed on said cathode surface to providefresh in situ deposited Cu.
 25. In a process according to claim 17wherein said granular Cu is intermittently formed on said cathodesurface to provide fresh in situ deposited Cu.
 26. In a processaccording to claim 17 wherein said granular Cu cathode surface isperiodically regenerated by anodic polarization followed by saidelectrodepositing Cu ions forming granular Cu on said cathode surface insitu to provide fresh in situ deposited Cu.